As used hereinafter, the word "xerography" is used to denote any imaging process wherein there is formed a pattern of electrostatic charges upon an image receptor. In "electrophotography" a uniform electrostatic charge is placed on a photoconductive insulating layer known as a photoreceptor, the photoreceptor is then exposed to a projected image of light and shadow whereby the surface charge on the areas exposed to light is dissipated, leaving an image-wise pattern of charges on the photoreceptor, known as an electrostatic latent image. In "laser xerography", a uniformly charged photoreceptor is discharged by the selective projection of a laser light source thereon, leaving a charge pattern on the photoreceptor. In "ionography" charged particles (air ions) are directly deposited, in an imagewise pattern, upon a conductively backed dielectric surface, known as a charge receptor. In each of these xerographic processes the electrostatic latent image is then developed, i.e. made visible, by the application of a finely divided particulate colorant, known as toner, in either powder or liquid form. The resultant developed image may then be transferred to a substrate, such as paper, and may be permanently affixed thereto by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
In the development step of the imaging process, the finely divided pigmented particles are brought into the vicinity of the electrostatic latent image by a transport mechanism, and will be attracted the image if they themselves bear an electrostatic charge opposite to that of the image areas on the charged surface. The toner particles used in xerography must become electrically charged in some manner either prior to or during the developing process in order to insure efficient and complete development of the image. When the toner is a dry powder, triboelectric charging (i.e. the appearance of static charges on insulating materials due to contact or friction) is the mechanism used. In the case of liquid toners, the finely divided particles suspended in a dielectric liquid become charged by virtue of their electrokinetic relationship with the surrounding liquid. Both of these particle charging phenomena are surface effects and are critically dependent upon traces of contamination and other factors affecting the nature of the surfaces involved.
Electrostatic images can be developed with dry powders by a number of different techniques. For example, a powder cloud may be generated adjacent the charged surface or the powder simply may be poured over the surface to be developed. In carrying out these mechanical operations, triboelectrification occurs and some of the particles acquire an electrical charge opposite in polarity to that of the image and hence are held on the image. For better control of the development process, the toner powder is mixed with a much coarser, granular, carrier material, and the mixture is cascaded, i.e. caused to flow, over the charged surface. In brush development, a carrier brush of mechanical or magnetic form transports the toner across the image area while simultaneously giving the toner the proper electrical charge. The carrier material selected for use with a given toner powder material must produce a triboelectric charge on the surface of the toner powder particles opposite in polarity to that of the image to be developed. Liquid development is usually effected by immersing the charged surface in an insulating liquid containing toner particles suspended therein.
Although a variety of materials can be used to develop xerographic images, such materials must be formulated to exacting standards to provide the specific physical properties required by the selected developing, transfer and fixing techniques employed and the requirements established for the final printed image. In general, a satisfactory powder developer material must have a number of attributes, some of which are: it should have a uniform chemical composition; it must be pulverizable or otherwise dispersible into fine particles and have a narrow optimum particle size distribution; it must have the proper color, color intensity and color density, and the proper transparency or opacity; it must be capable of accepting and retaining electrical charges of the correct sign; it should have no adverse effects on the environment nor should it adversely affect the charge receptor surface; it should have the proper characteristics for being fixed to a copy sheet, e.g. a melting point within the proper range for heat fixing or sufficient solubility for solvent vapor fixing; it should be easily cleaned from the image receptor without sticking or streaking; it should not agglomerate in storage; it should have an adequate shelf life: and, perhaps most important, it should be reproducible. If carrier particles are used with it to impart the proper triboelectric charge, the carriers must also be designed to satisfy exacting specifications.
An ideal liquid developer would have many of the same attributes as toner powder, such as color, surface charge magnitude and polarity, shelf life and reproducibility. Additionally, it should have good dispersibility, have the ability to maintain stability in solution, and be self fixing upon evaporation of the liquid carrier. The liquid medium should have a high volume resistivity so that the rate of destruction of the electrostatic image is minimized, a high dielectric constant and a high vapor pressure for quick drying. It should also be nontoxic, odorless, have a high flash and boiling point (i.e. nonflammable), have no solvent action on toner, have a specific gravity equal to or greater than that of the dispersed toner, have no reaction with the charge receptor surface, and be compatible with additive control agents (e.g. fixing and charge control).